Dawnward shift of the dayside O⁺ outflow distribution: The importance of field line history in O⁺ escape from the ionosphere

Abstract

[1] Observations and modeling of thermal upwelling O + in the topside ionosphere are used to demonstrate that the well-known dawnward shift of energetic escaping fluxes at higher altitudes is the result of the expansion and contraction of low energy plasma on magnetic field lines and noon focused energization mechanisms. Recent research has shown that magnetospheric impacts of ion outflow are dependent on the local time source location. However, most modelers assume that the peak escaping O + flux on the dayside is coincident with energy inputs from the magnetosphere associated with the cusp. The dawnward offset has been observed by multiple spacecraft and most recently in a new DMSP database of low altitude upwelling O + . The documented control of the cusp orientation with the Y component of the interplanetary magnetic field is generally a smaller effect than the observed dawnward bias. The dawnward offset is largest for thermal O + during geomagnetically quiet intervals and smallest for energetic escaping O + during active intervals. The present study investigates the efficacy of precipitating electrons, solar irradiance and neutral winds on dayside upwelling O + . An ionosphere model is used to show that solar irradiance causes significant upwelling O + on dawn flux tubes and when combined with noon focused energization mechanisms, establishes the observed dayside escaping energetic O + flux distribution. Our analysis also reveals that, during geomagnetically quiet intervals, high latitude neutral winds may be as important as electron precipitation in establishing the dayside distribution of upflowing thermal O + .

An assessment of the role of soft electron precipitation in global ion upwelling

“…The first of these processes is the upwelling of the cold O + ionospheric ions into the upper thermosphere. Upwelling associated with the rapid onset of bulk field‐aligned upward O + flux at sunrise at midlatitudes driven by photoionization was discovered early in the space age [e.g., Evans , ; Redmon et al ., ]. Upwelling of O + was subsequently observed at auroral and polar cap latitudes at all local times [e.g., Loranc et al ., ].…”

“…Recently, Redmon et al . [] demonstrated that the well‐known dawnward bias in escaping energetic O + fluxes is the result of peak O + upwelling just after dawn [ Evans , ] and noon‐focused energization mechanisms associated with the cusp. Redmon et al .…”

“…Redmon et al . [] compared observations of upwelling ions on Defense Meteorological Satellite Program (DMSP) with models of the plasma on magnetic field lines as they moved in and out of sunlight and regions of electron precipitation. Details of the modeling framework that uses the FLIP code are given in Redmon et al .…”

“…Details of the modeling framework that uses the FLIP code are given in Redmon et al . []. These investigations were made in dynamic boundary‐related coordinates (DBRL), which reduces ambiguities associated with the motion of the auroral zone in geographic coordinates.…”

“…Redmon et al . [] found that, on the dayside, electron precipitation plays a relatively minor role in driving upwelling O + ions except near noon, where it plays a supporting role. In this paper we extend the Redmon et al .…”